Display device and fabrication method thereof

ABSTRACT

The present invention provides a display device which forms thin film transistor circuits differing in characteristics from each other on a substrate in mixture and a fabrication method of the display device. On a glass substrate having a background layer which is formed by stacking an SiN film and an SiO 2  film, a precursor film which is constituted of an a-Si layer or a fine particle crystalline p-Si layer is formed and the implantation is applied to the precursor film. Here, an acceleration voltage and a dose quantity are adjusted such that a proper quantity of dopant is dosed in the inside of the precursor film. When the precursor film is melted by laser radiation, the dopant dosed in the precursor film is activated and taken into the precursor.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of nonprovisional U.S. application Ser.No. 11/590,882 filed on Nov. 1, 2006. Priority is claimed based on U.S.application Ser. No. 11/590,882 filed on Nov. 1, 2006, which claims thepriority of Japanese Application 2005-328865 filed on Nov. 14, 2005, allof which is incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a fabrication method of aflat-panel-type display device, and more particularly to a displaydevice which can form a large number of thin film transistors whichdiffer in operational characteristics on a substrate with small numberof steps and a fabrication method thereof.

Flat-panel-type display devices adopting various methods such as adisplay device capable of performing a high-definition color display fora notebook type computer or a display monitor, a liquid crystal displaydevice which uses a liquid crystal panel as a display panel for a mobilephone, an organic electroluminescence display device (organic EL displaydevice) which uses electroluminescence (particularly organicelectroluminescence) elements or a field emission type display device(FED) which uses field emission elements have been already put intopractice or in the process of studies for putting the display deviceinto practice.

With respect to the flat-panel-type display device, there has beendeveloped a so-called system-in-panel which directly builds a displayregion on which a large number of pixels which are constituted of a thinfilm transistor circuit are arranged in a matrix array and peripheralcircuits (including for example, a scanning signal drive circuit, avideo signal drive circuit for driving the pixels and other peripheralcircuits which are arranged around the display region) and the like inan insulating substrate made of glass or the like. The transparentinsulating substrate in which various kinds of thin film transistorcircuits are built is also referred to as a thin film transistor (TFT)substrate or an active matrix substrate, wherein in general, the thinfilm transistors having various characteristics are built in thesubstrate using a low-temperature poly silicon semiconductor film. Thesubstrate in which the thin film transistors are built in is alsoexpressed as the TFT substrate or simply as the substrate in theexplanation made hereinafter.

In building the pixel circuits which form the display region on the samesubstrate which constitutes the flat-panel-type display device and, atthe same time, in building a large number of thin film transistorcircuits including peripheral circuits (for example, the scanning signaldrive circuit, the video signal drive circuit and other peripheralcircuits) in a periphery of the substrate, channel regions which havethreshold voltages which differ in response to operational performancesof the respective circuits are formed on semiconductor layers of thesame substrate.

As one method, there has been proposed a method in which a semiconductorlayer of a thin film transistor forming portion of the circuit whichdoes not require a high speed operation is formed of a poly silicon(p-Si) layer (having a relatively large particle size) (for example,formed by annealing using an excimer laser (ELA) using an amorphoussilicon (a-Si) layer or a fine crystal poly-silicon layer as aprecursor), and a channel region of the circuit which requires ahigh-speed operation is selectively pseudo-single-crystallized to form apseudo-single-crystalline silicon semiconductor layer using a solidlaser, a continuous oscillation laser or the like. Here, thepseudo-single-crystalline silicon semiconductor layer implies, althoughdescribed in detail later, a semiconductor layer which grows relativelylarge crystals (having a strip-like shape, for example) compared tousual poly-silicon crystals which constitute so-called granular crystalsbut are not regarded as single crystals.

When the TFTs which differ in characteristics, that is, the TFTs whichare built in the poly-silicon semiconductor film formed by a techniquesuch as ELA, and the TFTs which are built in thepseudo-single-crystalline silicon semiconductor layer formed by using asolid laser, a continuous oscillation laser or the like are allowed tocoexist on the same substrate, it is necessary to control thecharacteristics (mainly threshold voltages) of the respective TFTs.

Here, patent documents which disclose the related art on thepseudo-single-crystals, for example, J-P-A-2002-222959 (patent document1), J-P-A-2003-124136 (patent document 2), J-P-A-2003-086505 (patentdocument 3) can be named.

SUMMARY OF THE INVENTION

As described above, the threshold voltage is, in general, controlled bythe method in which impurities (also referred to as dopants) are dosedinto the channel portion by ion implantation and the region where theimpurities are dosed is controlled in combination with aphotolithography step (a forming method which uses exposure and etchingprocessing).

However, in building the large number of thin film transistors whichdiffer in characteristics (threshold values) in the siliconsemiconductor layer on the common substrate, compared to the case inwhich the thin film transistors having the same characteristics arebuilt in the silicon semiconductor layers, photolithography steps andthe implantation steps are largely increased and hence, facilities andtime necessary for the fabrication are increased thus lowering theso-called throughput.

For example, to consider the thin film transistor of a single channel(only one of n-type or p-type), when it is necessary to make thethreshold value of the thin film transistor which uses the usualpoly-silicon (p-Si) in the pixel and the threshold value of the thinfilm transistor which uses the pseudo-single-crystals in the drivecircuit different from each other, one thin film transistor (forexample, the thin film transistor in the pixel) is masked by aphotolithography step and the implantation is applied to the channelregion of the thin film transistor of the pseudo-single-crystals.

Also in case that the thin film transistor is formed of a C-MIS(Complementary Metal insulator Semiconductor) (here, the MIS being usedas a concept which includes a MOS), the n-type thin film transistors andthe p-type thin film transistors exist in mixture and hence, it isnecessary to perform the photolithography steps and the implantationsteps for making the threshold values of the n-type thin filmtransistors and the p-type thin film transistors different from eachother.

Accordingly, it is an object of the present invention to provide adisplay device which can form thin film transistor circuits which differin characteristics from each other on a substrate in mixture and afabrication method of the display device.

According to the present invention, an amorphous silicon (a-Si) layer ora fine crystalline poly-silicon (p-Si) layer is used as a precursor filmand ion implantation is applied to the precursor film. Here, anacceleration voltage and a dose quantity are adjusted to dose a properquantity of dopant into the inside of the precursor film.

When the precursor film is melted by laser beam irradiation, the dopantcontained in the inside of the precursor film is activated and is takeninto the inside of the film. Further, the activation is performedsimultaneously with the crystallization and it is possible to obtain anactivation ratio of approximately 100%.

A quantity of the dopant dosed into the film is calculated by conversionwith respect to the activation ratio of 100% and may be smaller than aconventional dose quantity. Assume the activation ratio based on theconventional method as 10%, and a channel doped quantity is 1E¹²/cm² andthe doped quantity which actually contributes as the dopant isapproximately 1E¹¹/cm². On the other hand, in performing the ionimplantation before crystallization, the implantation condition may beset such that the dose quantity of approximately 1E¹¹/cm² is dosed.

The dopant is taken into the inside of the film only at a portion of thefilm to which the laser beams are radiated. Accordingly, it is possibleto obtain an advantageous effect as same as the advantageous effect ofthe method which performs the ion implantation only on the necessaryportion using a photolithography step.

Although the dopant dosed into the non-crystallized portion is partiallyactivated by annealing which is performed in a succeeding step, the dosequantity in the film is originally small and hence, the dopant does notconstitute the change of characteristics (fluctuation of a thresholdvalue).

To describe specific constitutional examples of the display device andthe fabrication method of the display device according to the presentinvention for achieving the above-mentioned object, they are as follows.

First of all, a fabrication method of a display device which forms afirst thin film transistor which is formed in a first region and has afirst threshold value, and a second thin film transistor which is formedin a second region and has a second threshold value which differs fromthe first threshold value on a substrate which constitutes the displaydevice is a fabrication process which includes:

a semiconductor film forming step which forms a semiconductor film;

a first impurity implanting step for implanting first impurities for athreshold value control in the semiconductor film which is formed in thefirst region and the second region;

a first crystallizing step for performing the crystallization of thesemiconductor film and the activation of the first impurities in thefirst region and the second region by applying heat treatment to thesemiconductor film in the first region and the second region;

a second impurity implanting step for implanting second impurities for athreshold value control in the semiconductor film in the first regionand the second region after the first crystallizing step; and

a second crystallizing step for performing the crystallization of thesemiconductor film of the second region and the activation of the secondimpurities by applying heat treatment only to the semiconductor film inthe second region out of the semiconductor films in the first region andthe second region.

Further, a display device according to the present invention which formsa first thin film transistor which is formed in a first region and has afirst threshold value, and a second thin film transistor which is formedin a second region and has a second threshold value which differs fromthe first threshold value is characterized in that

first impurities and second impurities are implanted into both of achannel region of the first thin film transistor and a channel region ofthe second thin film transistor;

the first impurities have a large activation ratio (90% or more in anumerical value) which activates substantially a total quantity ofimpurities in both of the channel region of the first thin filmtransistor and the channel region of the second thin film transistor;and

the second impurities have an activation ratio (50% or less in anumerical value) which activates substantially a half quantity ofimpurities at maximum in the channel region of the first thin filmtransistor and has a large activation ratio (90% or more in a numericalvalue) which activates substantially a total quantity of impurities inthe channel region of the second thin film transistor.

Further, in the fabrication method according to the present invention,the first crystallizing step may be a step in which the crystallizationis performed by radiating laser beams (gas laser beams, solid laserbeams or the like) to the semiconductor film, a step in which thecrystallization is performed by radiating excimer laser beams or solidlaser beams to the semiconductor film, or a step in which thecrystallization is performed by growing a solid phase by heating thesemiconductor film.

Further, in the fabrication method according to the present invention,the second crystallizing step may be any one of a step in which thecrystallization is performed by radiating laser beams to thesemiconductor film, a step in which the crystallization is performed byradiating continuous oscillation laser beams to the semiconductor film,a step in which the crystallization is performed by radiating continuousoscillation laser beams to the semiconductor film while modulating thelaser beams to pulses, and a step in which while continuous oscillationlaser beams are radiated to the semiconductor film, the scanning of thecontinuous oscillation laser beams is performed by moving at leasteither one of a spot of the continuous laser beams or a substrate onwhich the semiconductor film is formed thus forming strip-like crystals.

In the fabrication method of the present invention, the second impurityimplanting step may perform the implantation such that a peak positionof the concentration of the second impurities is arranged outside thesemiconductor film. Due to such a constitution, it is possible todecrease a dose quantity in the second impurity implanting step smallerthan a dose quantity in the first impurity implanting step.

Further, in the fabrication method of the present invention, at leastone of the first impurities and the second impurities may be implantedwithout through an insulation film. On the other hand, at least one ofthe first impurities and the second impurities may be implanted throughthe insulation film.

Alternatively, after at least one out of the first impurities and thesecond impurities is implanted through the insulation film, theinsulation film may be removed before the crystallization of thesemiconductor film. Further, the insulation film is removed after thefirst impurities are implanted through the insulation film and, then,after the removal of the insulation film, a surface oxide film is formedon a surface of the semiconductor film, and after the formation of thesurface oxide film, the first crystallizing step is performed.

Further, in the display device of the present invention, the activationratio of the second impurities in the channel region of the first thinfilm transistor may be approximately ⅓ (equal to or less than 30%numerically) at the maximum implanted quantity, and the channel regionof the first thin film transistor is formed of a semiconductor film madeof granular crystal or fine crystal.

Further, in the display device of the present invention, the channelregion of the second thin film transistor is formed of a semiconductorfilm made of strip-like crystal.

Further, in the display device of the present invention, a peak positionof the concentration of the second impurities may be positioned awayfrom the semiconductor film which constitutes the channel region.

Further, in the display device of the present invention, a dose quantityof the first impurities may be set larger than a dose quantity of thesecond impurities. Here, the first impurities and the second impuritiesmay be equal or different from each other.

Further, the present invention is not limited to the above-mentionedconstitutions and can be properly changed without departing from thetechnical concept of the present invention.

According to the present invention, a region in which pseudo singlecrystallization is performed is selectively set only to a portion inwhich the thin film transistor of the required high mobilitycharacteristics is built. Then, in combination with the laser beamirradiation step, a required dopant is selectively dosed only into theprecursor film of the portion where the pseudo-single-crystal thin filmtransistors are arranged. Although the dopant is dosed also into aportion to which the laser beams are not radiated, a dose quantity issmall and hence, a threshold value is not changed. Even when kinds ofthin film transistors (n-MOS, p-MOS) are increased, it is possible tocope with the increase of the kinds of thin film transistors byrepeating the ion implantation and the crystallization. Accordingly, itis unnecessary to increase photolithography steps thus improving theso-called throughput.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1A to FIG. 1W describe a flow chart showing fabrication steps of athin film transistor for explaining an embodiment 1 of a fabricationmethod of a display device according to the present invention in order;

FIG. 2A to FIG. 2J describe a flow chart showing fabrication steps of athin film transistor for explaining an embodiment 2 of a fabricationmethod of a display device according to the present invention in order;

FIG. 3A to FIG. 3F describe a flow chart showing fabrication steps of athin film transistor for explaining an embodiment 3 of a fabricationmethod of a display device according to the present invention in order;

FIG. 4 is an explanatory view of a first example when the ionimplantation of impurities in the fabrication method of the displaydevice according to the present invention is performed;

FIG. 5 is an explanatory view of a second example when the ionimplantation of impurities in the fabrication method of the displaydevice according to the present invention is performed;

FIG. 6 is an explanatory view of a third example when the ionimplantation of impurities in the fabrication method of the displaydevice according to the present invention is performed;

FIG. 7 is an explanatory view of a concentration profile of a dopanttaken into the crystallized film after the crystallization by laserbeams;

FIG. 8 is a view for explaining the difference in a concentrationprofile of a dopant in the depth direction of a silicon film in anactual product;

FIG. 9 is an explanatory view of a case in which the ion implantation isperformed with a precursor film covered with an insulation film and alsois a view corresponding to the above-mentioned embodiment 2;

FIG. 10 is a plan view for schematically explaining one example of athin film transistor substrate which constitutes a display device of thepresent invention; and

FIG. 11 is a schematic view for explaining a liquid crystal displaydevice of an embodiment of a display device according to the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention are explained indetail in conjunction with drawings showing the embodiments.Hereinafter, a substrate which forms a semiconductor layer is formed ofa glass substrate.

Embodiment 1

FIG. 1A to FIG. 1W are cross-sectional views for explaining anembodiment 1 of a fabrication method of a display device according tothe present invention and shows a fabrication process flow of an n-MOStop gate thin film transistor. Here, in order to explain the fabricationprocess flow by comparing the n-MOS top gate thin film transistor with athin film transistor having different characteristics which is formed onthe same substrate, a fabrication process of the thin film transistor ofhigh mobility (high-performance thin film transistor) at a left side ineach drawing and a fabrication process of the usual thin film transistorat a right side in each drawing are shown in parallel to each other.

FIG. 1A . . . An SiN layer 102 and an SiO₂ layer 103 are formed as abackground layer on a glass substrate 101, and an amorphous silicon film(a-Si) film 104 is formed on the SiN layer 102 and the SiO₂ layer 103 bya plasma CVD method. The heat treatment is applied to the stackedstructure to remove hydrogen in the inside of the a-Si film 104.

FIG. 1B . . . Into the a-Si film 104, a dopant 105 (mainly B⁺) forcontrolling a threshold voltage (Vth) of a n-MOS thin film transistor(TFT) using a polycrystalline (p-Si) semiconductor film is implanted(first implantation).

FIG. 1C . . . After implanting the dopant 105, excimer laser beams 106are radiated to reform the a-Si film into a p-Si film 107 (beingpolycrystallized, excimer laser annealing: ELA). Here, the activation issimultaneously applied to the dopant which is dosed into the a-Si film104 which functions as a precursor film.

FIG. 1D . . . The p-Si film 107 is formed. Here, the dopant 105 which isimplanted for controlling the threshold voltage (Vth) is activated bysubstantially 100%.

FIG. 1E . . . A dopant 108 for controlling a threshold voltage of ann-MOS pseudo-single-crystalline thin film transistor (TFT) is implantedinto the p-Si film 107 (second implantation).

FIG. 1F . . . The pseudo-single-crystallization is performed byannealing a predetermined region of the p-Si film 107 with solid laserbeams 109 or the like. The polycrystallization and the activation of thedopant 108 are simultaneously performed. The activation of the dopant108 is performed only with respect to a region which is annealed withlaser beams. An arrow indicates the scanning direction of laser beamsfor annealing. An arrow indicates the scanning direction of laser beamsfor annealing.

FIG. 1G . . . The region which is annealed by the solid laser beamsconstitutes a pseudo-single-crystalline silicon semiconductor film 110.

FIG. 1H . . . After finishing the crystallization, thepseudo-single-crystalline silicon semiconductor film 110 is formed in anisland-like shape thus forming a channel layer 112 formed of apolycrystalline silicon thin film transistor and a channel layer 111made of pseudo-single-crystalline silicon thin film transistor.

FIG. 1I . . . An SiO₂ film 113 which constitutes a gate insulation filmis formed by a plasma CVD method on the channel layer 111 of thepseudo-single-crystalline silicon thin film transistor and the channellayer 112 of the polycrystalline silicon thin film transistor which areformed in an island shape.

FIG. 1J . . . A gate metal layer 114 which becomes gate lines andcapacitance lines is formed.

FIG. 1K . . . A photo resist 115 is applied to the gate metal layer 114and forming is performed to allow the photo resist 115 to remain atpredetermined positions by a photolithography step.

FIG. 1L . . . A gate electrode 116 is formed by forming the gate metallayer 114 by etching. Here, a width of the gate electrode 116 is setsmaller than a width of the photo resist 115 by side etching.

FIG. 1M . . . An implantation (P⁺) for forming source/drain regions isperformed using the photo resist 115 as a mask.

FIG. 1N . . . Source/drain regions 118 are formed in the semiconductorfilm.

FIG. 1O . . . To form LDD (Light Doped Drain) regions, the impurities(P⁺) 119 of low concentration are implanted to the whole source/drainregions 118 using the gate electrode 116 as a mask.

FIG. 1P . . . The LDD regions 120 are formed in the source/drain regions118 by the implantation of the impurities (P⁺) 119.

FIG. 1Q . . . An interlayer insulation film 121 is formed and annealingtreatment for activating the implanted impurities is performed.

FIG. 1R . . . Contact holes 122 are formed in the interlayer insulationfilm 121 by a photolithography step. The contact holes 122 are formed ina state that the contact holes 122 reach the source/drain regions 118.

FIG. 1S . . . A barrier layer 123, an aluminum layer 124 and a cap layer125 are sequentially stacked thus forming a source/drain line 133. Thesource/drain line 133 is connected with the source/drain regions 118 viathe contact holes 122.

FIG. 1T . . . The source/drain line 133 is formed by a photolithographystep thus separating a source electrode 134 and a drain electrode 135.Here, for facilitating the explanation, the source electrode isindicated by 134 and the drain electrode is indicated by 135.

FIG. 1U . . . A passivation film 126 is formed in a state that thepassivation film 126 covers the source electrode 134 and the drainelectrode 135 which are separated from each other.

FIG. 1V . . . As conductive layers, a contact hole 127 with atransparent electrode made of ITO or the like and opening portions forpads not shown in the drawing are formed by a photolithography step.

FIG. 1W . . . An ITO film which is connected with the source electrode134 via the contact hole 127 and is connected with line terminals viathe opening portions for pads not shown in the drawing is formed on thepassivation film 126 and, ITO films which are respectively connectedwith a source and a drain electrode of the thin film transistorrespectively are formed by a photolithography step.

Due to the above-mentioned process of the embodiment 1, it is possibleto form the thin film transistors having different characteristics fromeach other which constitute channels being controlled at differentthreshold voltages by silicon semiconductor films which have differentcrystals on the same substrate.

According to the fabrication method of the embodiment 1, the thin filmtransistor which possesses the normal mobility constitutes a pixelcircuit in the pixel region on the active matrix substrate of thedisplay device. Then, various circuits are constituted by using the thinfilm transistor which possesses high mobility in the drive circuitregion which requires the high-speed data processing such as datadriving circuits. Accordingly, it is possible to constitute the displaydevice having the desired display performance without giving rise to theincrease of the number of steps.

Embodiment 2

FIG. 2A to FIG. 2J are cross-sectional views for explaining anembodiment 2 of a fabrication method of a display device according tothe present invention and shows a fabrication process flow of an n-MOStop gate thin film transistor in the same manner as the embodiment 1.

In this embodiment 2, before implanting the impurities, an insulationfilm is formed over the semiconductor and the implantation is performedthrough the insulation film. Such an insulation film may be made ofSiO₂, for example. This insulation film allows the impurities which areimplanted to pass therethrough and, at the same time, has a function ofprotecting the semiconductor to be implanted with impurities from thecontamination and hence, the insulation film may be also referred to asan implantation through film or a contamination prevention film. Theformation of the insulation film on the semiconductor film is performedbefore either one of the first-time implantation for controllingthreshold value or the second-time implantation for controllingthreshold value or before both of these processes. Here, it ispreferable to perform the removal of the insulation film after theimplantation. Hereinafter, the explanation is made by focusing on theprocess which makes the embodiment 2 differ from the embodiment 1.

FIG. 2A . . . An SiN layer 102 and an SiO₂ layer 103 are formed as abackground layer on a glass substrate 101, and an amorphous silicon(a-Si) film 104 is formed on the SiN layer 102 and the SiO₂ layer 103and, further, a SiO₂ film 131 is formed by a plasma CVD method. The heattreatment is applied to the stacked structure to remove hydrogen in theinside of the a-Si film 104.

FIG. 2B . . . A dopant 105 (mainly B⁺) for controlling a thresholdvoltage (Vth) of an n-MOS thin film transistor (TFT) using apolycrystalline (p-Si) semiconductor film is implanted into the a-Sifilm 104 through the SiO₂ film 131 (first implantation).

FIG. 2C . . . The a-Si film 104 is exposed by removing the SiO₂ film131.

FIG. 2D . . . After exposing the a-Si film 104, in the same manner asthe process shown in FIG. 1C in the embodiment 1, excimer laser beams106 are radiated to reform the a-Si film into a p-Si film 107 (beingpolycrystallized). Here, the activation is simultaneously applied to thedopant which is dosed into the a-Si film 104 which functions as aprecursor film.

FIG. 2E . . . The p-Si film 107 is formed. Here, the dopant 105 which isimplanted for controlling the threshold voltage (Vth) is activated by100%. This process is substantially equal to the process shown in FIG.1D.

FIG. 2F . . . An SiO₂ film 131 similar to the SiO₂ film 131 in FIG. 2Ais formed in a state that the SiO₂ film 131 covers the p-Si film 107.When necessary, heat treatment is applied to the silicon film 131 toremove hydrogen in the a-Si.

FIG. 2G . . . A dopant 108 for controlling a threshold voltage of ann-MOS pseudo-single-crystalline thin film transistor (TFT) is implantedinto the p-Si film 107 through the SiO₂ film 131 (second implantation).

FIG. 2H . . . The p-Si film 107 is exposed by removing the SiO₂ film131.

FIG. 2I . . . The pseudo-single-crystallization is performed byannealing a predetermined region of the p-Si film 107 with solid laserbeams 109 or the like. The polycrystallization and the activation of thedopant 108 are simultaneously performed. The activation of the dopant108 is performed only with respect to a region which is annealed withlaser beams. An arrow indicates the scanning direction of the laserbeams for annealing.

FIG. 2J . . . The region which is annealed by the solid laser beamsconstitutes a pseudo-single-crystalline silicon semiconductor film 110.Hereinafter, by performing the process shown in FIG. 1H to FIG. 1W, itis possible to form the thin film transistors having differentcharacteristics from each other which constitute channels beingcontrolled at different threshold voltages by silicon semiconductorfilms which have different crystals on the same substrate.

According to the fabrication method of the embodiment 2, the thin filmtransistor which possesses the normal mobility constitutes a pixelcircuit in the pixel region on the active matrix substrate of thedisplay device. Then, various circuits are constituted by using the thinfilm transistor which possesses high mobility in the drive circuitregion which requires the high-speed data processing such as datadriving circuits. Accordingly, it is possible to constitute the displaydevice having the desired display performance without giving rise to theincrease of the number of steps.

The above-mentioned embodiment 2 has been explained such that withrespect to both of the respective first-time implantation and thesecond-time implantation, the insulation film is formed before theimplantation and the insulation film is removed after the respectiveimplantations. However, as mentioned previously, the insulation film maybe formed only one of the first-time implantation and the second-timeimplantation and the insulation film may be removed after theimplantation. Further, the insulation film which is formed before thefirst-time implantation may not be removed and may be held until thesecond implantation is completed.

Embodiment 3

FIG. 3A to FIG. 3F are cross-sectional views for explaining anembodiment 3 of a fabrication method of a display device according tothe present invention and shows a fabrication process flow of an n-MOStop gate thin film transistor in the same manner as the embodiment 1.

In the embodiment 3, when the insulation film which is explained in FIG.2D in the embodiment 2 is removed, a thin oxide film (a surface oxidefilm) 132 is formed on a surface of the a-Si film 104 before the ELAprocess. The surface oxide film 132 can be formed by, for example, ozoneoxidation, oxygen plasma oxidation or the like. Hereinafter, theexplanation is mainly made with respect to the process which makes thisembodiment 3 differ from the second embodiment 2.

FIG. 3A . . . An SiN layer 102 and an SiO₂ layer 103 are formed as abackground layer on a glass substrate 101, and an amorphous silicon(a-Si) film 104 is formed on the SiN layer 102 and the SiO₂ layer 103and, further, an SiO₂ film 131 is formed on the amorphous silicon film104 by a plasma CVD method. The heat treatment is applied to the stackedstructure to remove hydrogen in the inside of the a-Si film 104.

FIG. 3B . . . A dopant 105 (mainly B⁺) for controlling a thresholdvoltage (Vth) of an n-MOS thin film transistor (TFT) using apolycrystalline (p-Si) semiconductor film is implanted into the a-Sifilm 104 through the SiO₂ film 131 (first implantation).

FIG. 3C . . . The a-Si film 104 is exposed by removing the SiO₂ film131.

FIG. 3D . . . A thin surface oxide film 132 is formed by applying theoxidation treatment to the a-Si film 104 by oxygen plasma oxidation.

FIG. 3E . . . A dopant 108 for controlling a threshold voltage of ann-MOS pseudo-single-crystalline thin film transistor (TFT) is implantedinto the p-Si film 107 through the surface oxide film 132 and the SiO₂film 131 (second implantation).

FIG. 3F . . . The pseudo-single-crystallization is performed byannealing a predetermined region of the p-Si film 107 with solid laserbeams 109 or the like. The polycrystallization and the activation of thedopant 108 are simultaneously performed. The activation of the dopant108 is performed only with respect to region which is annealed withlaser beams. An arrow indicates the scanning direction of the laserbeams for annealing. Hereinafter, by performing the process shown inFIG. 1G to FIG. 1W, it is possible to form the thin film transistorshaving different characteristics from each other which constitutechannels being controlled at different threshold voltages by siliconsemiconductor films which have different crystals on the same substrate.

In the embodiment 3, by forming the surface oxide film 132 on the SiO₂film 131, it is possible to prevent undesired impurities other than theimpurities which are implanted for controlling threshold value frombeing taken into thus controlling the threshold value of the thin filmtransistor to a desired value.

Usually, in performing the ELA crystallization, a natural oxide isformed on a surface of the a-Si film and hence, when the insulation film(the implantation through film or the contamination prevention film) isremoved before the ELA crystallization, a thickness of the natural oxidefilm on the surface is decreased thus giving rise to a possibility thatthe contamination occurs in performing the ELA crystallization.According to the embodiment 3, it is possible to obviate suchcontamination thus enabling the acquisition of the highly reliable thinfilm transistor.

Next, the relationship between the depth direction of the film thicknessand a concentration profile of the dopant due to the implantation isexplained. FIG. 4 is an explanatory view of the first example when theimplantation of impurities in the fabrication method of the displaydevice according to the present invention is performed. Further, FIG. 5is an explanatory view of the second example when the implantation ofimpurities in the fabrication method of the display device according tothe present invention is performed. Still further, FIG. 6 is anexplanatory view of the third example when the implantation ofimpurities in the fabrication method of the display device according tothe present invention is performed. In FIG. 4, FIG. 5, and FIG. 6, abackground film (an SiN film 302, a SiO₂ film 303) is formed on a glasssubstrate 301, and a precursor film 304 (an a-Si film or a p-Si film) isformed on the background film.

All of FIG. 4, FIG. 5 and FIG. 6 correspond to the process of theembodiment 1 in which the implantation is directly applied to theprecursor film 304 (the a-Si film or the p-Si film). FIG. 4 shows a casein which at the time of injecting the dopant, an implantation conditionis decided such that a maximum quantity of the concentration profile 305of the dopant in the depth direction (y direction) indicated by a heightin the x direction is within the background film. A dose quantity 308due to the implantation in the precursor film 304 indicated by meshedpoints in the drawing is activated to function as a dopant. The dosequantity 308 due to the implantation indicated by meshed points in thedrawing performs both of the crystallization and the activation andhence, 90% of more of the dose quantity 308 is activated. Accordingly, adopant quantity which is dosed into the precursor film 304 may be set toa small value.

FIG. 5 shows a case in which at the time of injecting the dopant, animplantation condition is decided such that a maximum quantity of theconcentration profile 307 of the dopant in the depth direction (ydirection) indicated by a height in the x direction is outside theprecursor film 304. A dose quantity 308 due to the implantation in theprecursor film 304 indicated by meshed points in the drawing isactivated to function as a dopant. The dose quantity 308 due to theimplantation indicated by meshed points in the drawing performs both ofthe crystallization and the activation and hence, 90% of more of thedose quantity 308 is activated. Also in this case, a dopant quantitywhich is dosed into the precursor film 304 may be set to a small value.

FIG. 6 shows a case in which at the time of injecting the dopant, animplantation condition is decided such that a maximum quantity of theconcentration profile 309 of the dopant in the depth direction (ydirection) indicated by a height in the x direction is within theprecursor film 304. A total dose quantity is set smaller than the totaldose quantity in the cases shown in FIG. 4 and FIG. 5. The dose quantity309 due to the implantation in the precursor film 304 indicated bymeshed points in the drawing is activated to function as a dopant. Thedose quantity 309 due to the implantation indicated by meshed points inthe drawing performs both of the crystallization and the activation andhence, 90% or more of the dose quantity 309 is activated.

FIG. 7 is an explanatory view of the concentration profile of the dopantwhich is taken into the crystallized film after the crystallization bylaser beams and the concentration profile of the dopant corresponds tothe concentration profile of the dopant shown in FIG. 4. In FIG. 7, abackground film (an SiN film 402, a SiO₂ film 403) is formed on a glasssubstrate 401, and a precursor film 404 (an a-Si film or a p-Si film) isformed on the background film. In the crystallization process after thedopant process, a maximum quantity of the concentration profile 405 iswithin the background film 403. With the radiation of laser beams, theprecursor film 404 is melted and hence, the dopant in the inside of theprecursor film 404 is taken into the crystallized film. Here, theconcentration profile 406 of the dopant in the depth direction in theinside of the crystallized film becomes uniform.

FIG. 8 is a view for explaining the difference in the dopantconcentration profile in the depth direction of the silicon film in anactual product. In FIG. 8, a background film (an SiN film 502, an SiO₂film 503) is formed on a glass substrate 501, and various regions areformed on the background film. Numeral 504 indicates a p-Si region andnumeral 507 indicates a pseudo-single-crystalline region. Numeral 505indicates an implantation concentration profile, numeral 506 indicatesan implantation concentration profile in the inside of the p-Si region504, and numeral 508 indicates the implantation concentration profile inthe pseudo-single-crystalline regions 507.

In FIG. 8, the region 504 to which the crystallization by laser beams isnot applied after performing the implantation exhibits the concentrationdistribution of the dopant indicated by numeral 506 in the depthdirection. To the contrary, in the region 507 where the crystallizationis selectively performed, the activation of the dopant is performedsimultaneously with the crystallization and hence, the concentrationprofile 508 is fixed in the depth direction.

FIG. 9 is an explanatory view of the case in which the implantation isperformed by covering the precursor film with an insulation film fromabove and the case corresponds to the case explained in conjunction withthe embodiment 2. In FIG. 9, on a glass substrate 201 on which an SiNfilm 202 and an SiO₂ film 203 are formed as a background film, an a-Sifilm or a p-Si film is formed as the precursor film 204, and aninsulation film (an implantation through film or a contaminationprevention film) 207 is formed in a state that the insulation filmcovers the background film. A dopant is dosed through the insulationfilm 207.

At the time of injecting the dopant, an implantation condition isdecided such that a maximum quantity of the concentration profile 205 ofthe dopant in the depth direction (y direction) indicated by a height inthe x direction is within the precursor film 204. A portion of dosequantity 208 due to the implantation in the precursor film 204 indicatedby meshed points in the drawing is activated to function as a dopant.

FIG. 10 is a plan view for schematically explaining one example of athin film transistor substrate which constitutes a display device of thepresent invention. The thin film transistor substrate (low-temperaturepoly-silicon TFT substrate) arranges a pixel region 602, peripheralcircuits (video signal drive circuits (a signal processing circuit 603,a horizontal-direction scanning circuit 604), a scanning signal drivecircuit (a vertical-direction scanning circuit 605), a booster circuitand other peripheral circuit 606), and an input pad 607 on a glasssubstrate 601 thereof.

In the signal processing circuit 603, the horizontal-direction scanningcircuit 604 and other peripheral circuit 606 which require a high-speedoperation, a thin film transistor which uses a pseudo-single-crystallinesilicon semiconductor in a channel region is formed. In the pixel region602 and the vertical-direction scanning circuit 605 which constituteother circuit parts, a thin film transistor which uses apoly-crystalline silicon semiconductor in a channel region is formed.However, the pseudo-single-crystalline silicon semiconductor may be alsoused in the vertical-direction scanning circuit 605 and the pixel region602. Further, usual polycrystal may be also used in the signalprocessing circuit 603, the horizontal-direction scanning circuit 604and other peripheral circuit 606. Here, in one circuit, a thin filmtransistor which uses a usual poly-crystalline silicon semiconductor anda thin film transistor which uses a pseudo-single-crystalline siliconsemiconductor may be used in mixture.

Embodiment 4

Next, the embodiment of the display device according to the presentinvention is explained as the fourth embodiment 4. FIG. 11 is aschematic view for explaining a liquid crystal display device whichconstitutes the embodiment of the present invention. On a glasssubstrate 5011, a plurality of pixel electrodes 5031 which are arrangedin a matrix array, circuits 5071 and 5111 which input display signals tothe pixel electrodes, and a group of other peripheral circuits 5180which is necessary for image display are formed and, thereafter, anorientation film OR 5190 is applied by a printing method thus forming anactive matrix substrate.

On the other hand, a color filter substrate which applies counterelectrodes 5212, color filters 5213 and an orientation film 5214 onto aglass substrate 5211 thereof is prepared, and the color filter substrateis overlapped to the active matrix substrate. Liquid crystal 5215 isfilled between the orientation films 5190, 5214 which face each other byvacuum injection and the liquid crystal is sealed by a sealing material5216. Thereafter, polarizers 5217, 5218 are respectively adhered toouter surfaces of the glass substrate 5011 and the glass substrate 5211.Then, a backlight 5219 is arranged on a back surface of the activematrix substrate thus completing the liquid crystal display device.

Here, although the explanation has been made with respect to the liquidcrystal display device which forms the color filters on the countersubstrate side of the active matrix substrate, the present invention isalso applicable to a liquid crystal display device of a type which formscolor filters on an active matrix substrate side of the countersubstrate. Further, although FIG. 11 shows the color filter substratewhich is formed by stacking the counter electrodes 5212, the colorfilters 5213 and the orientation film 5214 on the glass substrate 5211in this order, the color filter substrate may adopt the structure inwhich the color filters are formed on the glass substrate 5211, thecounter electrodes 5212 are formed on the color filters, and theorientation film 5214 is formed as an uppermost layer. The positionwhere the color filters are formed and the structure of the color filtersubstrate are not directly relevant to the technical concept of thepresent invention.

According to this embodiment, it is possible to directly form thepixels, the drive circuits which drive the pixels and other peripheralcircuits on the active matrix substrate corresponding to requiredcharacteristics of these parts and hence, it is possible to acquire theliquid crystal display device having the favorable display quality whichcan enlarge the pixel region and exhibits the high-speed operation andthe high resolution.

The present invention which has been explained heretofore is not limitedto the liquid crystal display device and may be applicable to an organicEL display device and other various active-matrix-type display devices.

Further, with respect to the threshold value control due to doping ofimpurities to the semiconductor layer of the channel region of the thinfilm transistor, B⁺ impurities are used for the n-type thin filmtransistor and P⁺ impurities are used for the p-type thin filmtransistor in the embodiment. However, the impurities which are doped inthe channel region are irrelevant to the decision of the conductive typeof the thin film transistor and hence, P⁺ impurities may be used for then-type thin film transistor and B⁺ impurities may be used for the p-typethin film transistor when necessary.

1. A fabrication method of a display device which forms a first thinfilm transistor which is formed in a first region and has a firstthreshold value, and a second thin film transistor which is formed in asecond region and has a second threshold value which differs from thefirst threshold value, the fabrication method comprising: asemiconductor film forming step which forms a semiconductor film; afirst impurity implanting step for implanting first impurities for athreshold value control in the semiconductor film which is formed in thefirst region and the second region; a first crystallizing step forperforming the crystallization of the semiconductor film and theactivation of the first impurities in the first region and the secondregion by applying heat treatment to the semiconductor film in the firstregion and the second region; a second impurity implanting step forimplanting second impurities for a threshold value control in thesemiconductor film in the first region and the second region after thefirst crystallizing step; and a second crystallizing step for performingthe crystallization of the semiconductor film of the second region andthe activation of the second impurities by applying heat treatment onlyto the semiconductor film in the second region out of the semiconductorfilms in the first region and the second region.
 2. A fabrication methodof a display device according to claim 1, wherein the firstcrystallizing step is a step in which the crystallization is performedby radiating laser beams to the semiconductor film.
 3. A fabricationmethod of a display device according to claim 2, wherein the firstcrystallizing step is a step in which the crystallization is performedby radiating excimer laser beams or solid laser beams to thesemiconductor film.
 4. A fabrication method of a display deviceaccording to claim 1, wherein the first crystallizing step is a step inwhich the crystallization is performed by growing a solid phase byheating the semiconductor film.
 5. A fabrication method of a displaydevice according to claim 1, wherein the second crystallizing step is astep in which the crystallization is performed by radiating laser beamsto the semiconductor film.
 6. A fabrication method of a display deviceaccording to claim 5, wherein the second crystallizing step is a step inwhich the crystallization is performed by radiating continuousoscillation laser beams to the semiconductor film.
 7. A fabricationmethod of a display device according to claim 5, wherein the secondcrystallizing step is a step in which the crystallization is performedby radiating continuous oscillation laser beams to the semiconductorfilm while modulating pulses.
 8. A fabrication method of a displaydevice according to claim 5, wherein the second crystallizing step is astep in which while continuous oscillation beams are radiated to thesemiconductor film, the scanning of the continuous oscillation laserbeams is performed by moving at least either one of a spot of thecontinuous laser beams or a substrate on which the semiconductor film isformed thus forming strip-like crystals.
 9. A fabrication method of adisplay device according to claim 1, wherein the second impurityimplanting step is a step in which the implantation is performed suchthat a peak position of the concentration of the second impurities isarranged outside the semiconductor film.
 10. A fabrication method of adisplay device according to claim 1, wherein a dose quantity into thesemiconductor film in the second impurity implanting step is smallerthan a dose quantity in the first impurity implanting step.
 11. Afabrication method of a display device according to claim 1, wherein atleast one of the first impurities and the second impurities is implantedwithout an insulation film.
 12. A fabrication method of a display deviceaccording to claim 1, wherein at least one of the first impurities andthe second impurities may be implanted through the insulation film. 13.A fabrication method of a display device according to claim 12, whereinafter at least one of the first impurities and the second impurities isimplanted through the insulation film, the insulation film is removedbefore the crystallization of the semiconductor film.
 14. A fabricationmethod of a display device according to claim 13, wherein the insulationfilm is removed after the first impurities are implanted through theinsulation film and, then, after the removal of the insulation film, asurface oxide film is formed on a surface of the semiconductor film, andafter the formation of the surface oxide film, the first crystallizingstep is performed.